The present invention relates to a gas separation apparatus, a front wall and a separation rotor thereof. The present invention especially relates to the modification of the separation rotor and/or the front wall of the gas separation apparatus in such a way that the liquid or liquid suspension separated by the separation rotor from a mixture containing gas and liquid or gas and liquid suspension may be recirculated as simply as possible back to a process flow as the gas separated from the liquid or liquid suspension is guided out of the process. The present invention may be applied in all such positions that gas separates from a process liquid flow to a certain spot in the flow, wherefrom the gaseous mixture, i.e. either a mixture of gas and liquid or gas and suspension, may be brought to the gas separation apparatus. It is especially advantageous to use a gas separation apparatus in accordance with the invention for separating, from the process flow, gas accumulated in the center part or some other cavity prevailing at a reduced pressure in rotary apparatuses. An advantageous rotary apparatus worth mentioning is a centrifugal pump or a specific gas separator.
The following description discusses both the prior art and the present invention more in detail in connection with centrifugal pumps. However, the invention is not intended to be limited to use with centrifugal pumps only, but the applications in connection with centrifugal pumps are merely shown as preferred embodiments of the invention.
There are previously known centrifugal pumps that are capable of separating gas. In those pumps the gas accumulates in front of the impeller of the pump forming a bubble from which the gas is discharged through openings in the backplate of the impeller to the cavity behind the impeller. Almost always some liquid or suspension to be pumped is entrained with the gas. The aim with the use of the vanes behind the backplate of the impeller is to attempt to separate liquid or suspension entering the rear side of the impeller from the separated gas in such a way that the liquid or suspension is returned around the outer rim of the backplate of the impeller to the liquid or suspension to be pumped and the gas is discharged along the pump shaft out of the pump.
It has, however, been shown in practice that in very many applications liquid or suspension is still entrained with the gas. In order to prevent the liquid or suspension to be pumped from being entrained with the gas to the suction apparatus, which may be, for example, a vacuum pump, a separate separation chamber has been arranged in connection with the so-called rear wall of the pump for a separation rotor. The mixture of gas and liquid or liquid suspension that has entered the cavity is, by means of the separation rotor, brought to such a vigorous rotational movement that practically speaking all liquid and possible solid material in the mixture gathers to the circumference of the separation chamber, from where it may be discharged and returned back to the liquid or suspension pumped or being pumped. A conventional structure comprises a gas separation rotor consisting of a hub of the separation rotor and radial or inclined vanes attached thereto. It is a typical feature of the gas separation rotors of the prior art that the intermediate spaces between the vanes (i.e. vane passages) are open from the hub to the outer rim so that the gas is allowed to flow from the vane passages substantially axially towards the gas discharge as easily as possible.
The discharge of liquid or suspension from the separation chamber is arranged along a separate channel arranged most usually outside the pump to take the liquid or suspension to the suction duct of the pump. The structure is, however, complicated and expensive to carry out.
On the other hand, there are known dynamic sealing arrangements used, for example, in centrifugal pumps, a substantial part of the sealing arrangements being an internal liquid cycle within the pump from a dynamic sealing chamber in connection with the rear wall of the pump back to the cavity behind the impeller of the pump. A dynamic seal is, as known, by its nature a seal, which is without any mechanical contact able to seal the centrifugal pump during the operation of the pump so that no liquid is allowed to flow along the shaft towards the bearing and the drive (on the right in the drawings). For example, U.S. Pat. No. 5,344,163 illustrates the structure and operational principle of the seal as well as the location of a conventional dynamic seal in the pump. The dynamic seal is thus located behind the volute of the pump in front of the pump bearing (seen from the direction of the suction duct) and in an annular chamber arranged in connection with a so-called rear wall of the pump, which chamber is in direct flow communication with the volute of the pump, where the impeller of the pump rotates. A rotating disc attached on the shaft of the pump divides the chamber into an impeller side cavity and a pump bearing-side cavity. The disc is provided with vanes on the side facing the bearing-side cavity, whereby it may also be called an expeller, while the other side of the disc is smooth. In case the annular chamber contains liquid, the vanes of the expeller tend to pump the liquid, first radially outwards, then around the outer rim of the expeller disc to the impeller side cavity of the chamber. Now, however, when the pump is in operation the pressure generated by the impeller to the volute of the pump effects to the opposite direction, whereby a balance is found where the liquid ring rotated by the expeller vanes neutralizes the pressure generated by the impeller and the pump is sealed in such a way that no liquid enters the shaft space in the bearing-side cavity of the chamber when the pump is in operation. However, when the pump is not in operation, the liquid to be pumped has free access around the rim of the expeller disc in the chamber to the shaft space and therethrough to the atmosphere, if it is not prevented in some suitable manner. For this purpose, a so-called static seal is used, which at its simplest is a rotary disc arranged round the shaft and pressed by the pressure of the liquid entering the shaft space against the counter surface preventing the flow of the liquid further.
A disadvantage relating to the dynamic sealing in view of the present problem, i.e. gas separation, is that it can be used neither with a gas-removing centrifugal pump nor with any other gas-separating rotary apparatus or apparatuses, because there is/are no opening(s) in the expeller disc of a conventional dynamic seal for the gas to be discharged.
One solution combining, in a way, features of a conventional gas separation chamber, of a gas separation impeller rotating therein and of an expeller of a dynamic seal for the purposes of gas separation is disclosed in publication WO-A1-90/13344 and in a partially sectional view in FIG. 1. The publication relates to a gas separator, in which a cage-like rotor 10 is used for generating such a vigorous centrifugal force field in the pulp of wood entering the apparatus that the gas in the pulp separates to the center of the apparatus. The separated gas is discharged from the center of the apparatus through openings 14 in the rotor disc 12 to the rear side of the disc and therefrom further through a gap between the hub 16 and the rear wall of the apparatus. In case pulp is entrained with the gas, a separation chamber 18 is arranged in connection with the rear wall of the apparatus, which chamber again is provided with a separation rotor attached on the shaft of the apparatus. The separation rotor comprises a hub 16, vanes 20 extending outwards therefrom and a solid annular disc 22 attached on the side of the vanes 20 facing the rotor 10, the disc dividing the separation chamber 18 to a front (rotor side) chamber 24 and a rear chamber 26. The operational principle of the separation apparatus is that the vanes 20 of the separation rotor swing the pulp that has entered within their reach to the outer circumference of the separation chamber so that no pulp is able to pass axially the separation apparatus. In other words, this far the principle has been the same as in the previously described gas-separating pump. Now, however, when the annular disc 22 attached to the vanes 20 of the separation rotor divides the separation chamber 18 into two parts, into the rear chamber 26 of which the vanes 20 pump the pulp, the pumped pulp may flow around the rim of the disc 22 to the front chamber 24. This has been facilitated so that a closing member 30 extends to the disc 22 from the rear wall 28 of the pump, the rear wall 28 of the pump acting as the front wall of the front chamber 24. The closing member 30 is used for closing the radial flow connection between the front chamber 24 and the shaft space, to which the separated mixture of gas and pulp first arrives. Correspondingly, openings 32 have been arranged, in the radial direction outside the closing member 30, through the rear wall 28 of the pump, the rear wall 28 acting as the front wall of the separation chamber 18, through which openings 32 the pulp entered to the front chamber 24 is returned to the space in front of the rear wall 28, in which rear vanes 34 of the disc 12 of the rotor 10 of the actual gas separation apparatus directs the pulp back to the process. Furthermore, it is worth noting that, in the apparatus in accordance with the publication, there are turbulence-generating vanes 36 on the surface of the disc 22 of the separation chamber 18, the surface facing the front chamber 24, by means of which vanes 36 clogging of the return openings 32 for pulp is prevented.
There are some disadvantages in the apparatus illustrated in FIG. 1. First of all, it is designed for relatively thick (high consistency) fiber suspensions, the flow dynamics of which considerably deviate from that of liquids and very low consistency suspensions of the present invention. For example, mere liquid or low consistency suspension rotates very easily in the separation chamber. While high consistency pulp requires rotary vanes generating turbulence to prevent the return openings from being clogged by pulp, the same vanes with mere liquid would intensify the rotation of the liquid in the separation chamber and would substantially harm the return of the liquid back to the volute of the pump.